Alzheimer's disease is associated with the abnormal deposits of amyloid beta protein (Ab) that occurs as fibrils within the cerebral neuropil. To characterize the way in which these fibrils assemble under different pH conditions, we have performed transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) on synthetic peptides that contain the motif KLVFFAE associated with beta-sheet formation. TEM images of negatively stained preparations provide information about the fibril width and morphology. STEM analysis of rapidly-frozen , unstained preparations provides a quantitative determination of the mass-per-length (MPL) and thus the numbers of beta-sheets within fibrils. The results demonstrate how the structures of fibrils grown from Alzheimer-related Ab peptides depend on the peptide length and on the growth conditions. We have shown that Ab (10-35) fibril morphology depends strongly on pH. MPL measurements of Ab (10-35) fibrils suggest the existence of a fundamental fibrillizing unit, or ?protofilament?, consisting of two layers of cross-beta sheet. Electron micrographs and MPL measurements of human Ab (1-42) fibrils also indicate a fundamental fibrillizing unit consisting of two beta-sheet layers. In contrast, electron micrographs and MPL measurements of rat Ab (1-42) fibrils show two distinct fibril morphologies with a single MPL value, corresponding to three layers of beta-sheet.